With the ever increasing growth of automated systems used in various types of industrial and medical systems, there is a need for new and improved sensors and signal processing apparatus for monitoring movements related to force, torque, speed, acceleration, contraction, expansion, deformation, displacement, and the like. There is also a need to miniaturize such sensors to make measurements not otherwise possible with large and bulky sensors. For example, when monitoring the displacements of small or fragile items, or when monitoring deformations of flexible membranes such as skin, it is important that the sensor mass, attachments, electrical connections, and its operation do not interfere with the movements being monitored that might otherwise significantly impact the accuracy of the measurements.
Such sensors and signal processing apparatus should preferably be subject to low manufacturing costs, not require high tolerance fits for moving parts, provide a sizable range of linear operation, and yet be relatively rugged.
Linear variable differential transformer (LVDT) type sensors are widely in use for making displacement measurements. A LVDT sensor is disclosed in the U.S. Pat. No. 5,216,364, issued on Jun. 1, 1993, entitled “Variable Transformer Position Sensor” includes mechanical structures that are useful for automotive shock absorbers but are too massive in design to readily miniaturized or to be applied to monitoring delicate items or flexible membranes. Miniaturized sensors based on the LVDT technology are disclosed in catalog publications by Micro-Epsilon entitled “Inductive Displacement Sensor and Linear Gaging Sensors,” and by Singer Instruments and Control, Ltd. entitled “SM Series LVTD.” These sensors include a primary or input winding and two secondary or output windings transformer coupled to a movable core.
The U.S. Pat. No. 5,497,147, issued on Mar. 5, 1996, and entitled “Differential Variable Reluctance Transducer,” and U.S. Pat. No. 5,777,467, issued on Jul. 7, 1998, and entitled “Miniaturized Displacement Transducer Assembly,” and publication by MicroStrain entitled “Differential Variable Reluctance Transducer” (DVTR) disclose sensors that include two output coils and depend upon the differential reluctance between the coils controlled by a movable core for monitoring displacements by using sine wave excitation and synchronous demodulation. The close operating tolerances of the sensor require a highly flexible core structure of super elastic material to achieve the free sliding fit between the core and the coils to be robust to mechanical damage.
The U.S. Pat. No. 3,891,918, issued on Jun. 24, 1975, and entitled “Linear Displacement Transducer Utilizing An Oscillator Whose Average Period Varies as a Linear Function of the Displacement,” includes a single coil winding or two windings and a core as a variable inductance. One or both of the sensor windings controls the frequency of an oscillator circuit in a well-known manner to provide signals for measuring displacements.
The sensors in the U.S. Pat. No. 5,216,364, and the Micro-Epsilon and the Singer publications, and in a publication by Analog Devices entitled “LVDT Signal Conditioner AD598 (Rev A)” and a publication by David S. Nyce of Revolution Sensor Company entitled “The LVDT a Simple and Accurate Position Sensor” dated August 2005 disclose arrangements where constant sinusoidal excitation signals are applied to the primary winding of frequencies of 20 Hz to 12 KHz, depending on the sensor, and wherein alternating current signals are induced in both secondary windings with amplitudes depending on the core position that are added or subtracted to generate measurement signals.
The Q (quality factor) of a coil is defined as the ratio of the inductive reactance to the resistance of the coil at a given frequency. Q is a measure of the efficiency of storing energy; the higher the Q the more efficient the coil. To increase the Q in the abovementioned sensors, either the frequency applied to the sensors is to be increased, or the sensor inductive reactance increased (by the number of coil turns squared), or the sensor internal resistance is decreased. However, the miniaturization does not scale well due to Q restraints. As the dimensions of these sensors are decreased, primarily by reducing the size of the wire, the internal resistance of the sensor coils increases significantly. It would be advantageous if the sensor design were not limited by Q restraints, allowing miniaturization by the use of smaller gauge wires with its inherent increase of internal resistance without materially impacting the sensor performance.
The sensor systems identified above disclose the continuous application of signals to the sensors and a continuous power supply for energizing the signal processing circuitry. It would be advantageous if an arrangement would be provided that such sensor systems would require substantial power draw only during signal and data processing. An arrangement for reducing power consumption is disclosed in the U.S. Pat. No. 6,433,629, issued Aug. 13, 2002, entitled “Micropower Differential Sensor Measurement,” wherein power for the system is temporarily developed from a transmitted magnetic field for a duration to allow a pulse to be applied to a Wheatstone sensor bridge circuit, a comparison made, and an RF signal transmitted for data processing. The disclosed power generating arrangement is time consuming requiring rectification and filtering of the magnetic field and the stabilization of the power supply before signal processing is initiated.
The pulse energization of the sensor Wheatstone bridge circuit arrangement, as disclosed in the U.S. Pat. No. 6,433,629, requires the comparison of signals based on a non-linear RC or LR exponentially curved decay rates.
Temperature variations or gradients place limitations on the absolute accuracy of variable impedance inductive sensors. For example, the intrinsic resistance of the windings of the sensor coils changes with variations in temperature. In addition, the permeability of metallic sensor cores changes with variations in temperature introducing changes in sensor inductance. In the U.S. Pat. No. 5,914,593, issued on Jun. 22, 1999, entitled “Temperature Gradient Compensation Circuit,” and United States Patent Application Publication No. 2005/0093537, published on May 5, 2005, entitled “Circuit for Compensating for Time Variation of Temperature in an Inductive Sensor,” such temperature sensitive changes in sensor impedance are electronically compensated by the use of a Wheatstone bridge sensor circuit configuration including the application of AC and DC signals to the bridge circuit to obtain temperature offset correction signals. It would be desirable if the sensors could be configured in circuit arrangements that provide self-temperature compensation.
In the field of medicine there is continual research and development for the design of new equipment for monitoring body volume changes to measure internal physiological properties, such as the chest for problems dealing with sleep apnea and the abdomen for pregnancy labors. The present solutions require the use of belt and/or vest type sensing arrangements. For sleep apnea the vests and belts surround the chest torso such as disclosed in many United States patents, of which the following are sample patents: U.S. Pat. No. 5,329,932, issued Jul. 19, 1994, entitled “Method of and Apparatus for Monitoring Respiration and Conductive Composition Used Therewith,” U.S. Pat. No. 6,142,953, issued Nov. 7, 2000, entitled “Respiratory Inductive Plethysmography Band Transducer,” U.S. Pat. No. 6,413,225, issued Jul. 2, 2002, entitled “Quantitative Calibration of Breathing Monitors with Transducers Placed on Both Rib Cage and Abdomen,” U.S. Pat. No. 6,461,307, issued Oct. 8, 2002, entitled “Disposable Sensor for Measuring Respiration,” and U.S. Pat. No. 6,551,252, issued Apr. 22, 2003, entitled “Systems and Method for Ambulatory Monitoring of Physiological Signs.” For pregnancy labors, the belts surround the abdomen such as disclosed in a Philips Medical Systems Nederland B. V. publication entitled “FM-2 Antepartum Portable Fetal Monitor.” Each of these apparatus is bulky and as a result may be relatively uncomfortable to wear for extended periods of time, particularly if required to wear them overnight. Furthermore, although the apparatus may be portable, they are cumbersome, and may interfere with daily activities and sleep.
The use of commercial strain gauges to measure deformations of the body was found unworkable in that the attachment of such strain gauges to the body interfered with the movements of the part of the skin to which the gauges were attached rendering their use questionable.
There is a need to replace these massive and cumbersome belts and vest apparatus that encircle the body or cover large portions of the torso, and avoid short-term and long-term patient discomfort that may accompany their use. The apparatus should preferably be attached and worn with minimal discomfort, allowing the patient a significant amount of freedom of movement without impacting the tests underway. The apparatus should also preferably have a high degree of sensitivity to allow the equipment to detect small changes, particularly when testing infants, and be capable of continued operation as the patient changes positions.
It would be advantageous if the connection of the sensing apparatus to the body could be made similar to a “Band-aid” tape type arrangement so that the attachment can be simplified and made by technicians, and that the associated monitoring equipment can be easily set up and maintained in operation.
Further it would be advantageous if the sensor could be subject to miniaturization so that appropriate electronics and transmission circuitry could be designed attached to the body for radio, infrared, etc, transmission of data to remote locations. It would also be advantageous for portability purposes if such sensors and circuitry could be powered by small wrist-watch type batteries and still perform for time periods needed to complete the tests before replacement is needed.
In addition, it would advantageous if the monitoring apparatus was adaptable for use over a wide variety of portions of the body for observing a wide variety of physiological problems.